You need to change the folder of the current graphene theme to allow you to be able to access the site again. Once you do this, you’ll be able to go into your dashboard and change the theme to the old theme you were using before Graphene. As for troubleshooting the theme, it may be a corrupt download. You may try downloading the theme and installing it again.

You just need to rename the folder, this will force WordPress to use the default theme. You can rename it to whatever you’d like. This will allow you to access your dashboard once again to make further changes.